\documentclass{article}

\usepackage{booktabs}
\usepackage{dcolumn}

\newcolumntype{d}{D{.}{.}{5.5}}
\newcolumntype{C}{>{\centering}p}

\begin{document}

\begin{table}[htp]
\caption{}
\centering
\begin{center}
    \begin{tabular}{p{1.25in}ddd}
    \toprule
    \multicolumn{1}{C{1in}}{\shortstack{Group}} & \multicolumn{1}{C{0.75in}}{\shortstack{Weight\\(g)}} &    \multicolumn{1}{C{1in}}{\shortstack{Glucose\\(mg/l)}} & \multicolumn{1}{C{2in}} {\shortstack{Glycosylated protein\\(nmol/mg protein)}}\\
    \midrule
    Control & 544.5 & 908.0 & 17.8\\
    Diabetic & 331.2{$^{***}$} & 5736.0{$^{***}$} & 32.10{$^{***}$}\\
    Diabetic+GT & 343.9{$^{***}$} & 4037.0{$^{***}\dagger\dagger$ } &   24.87{$^{***}$}\\
    \bottomrule
    \end{tabular}
\end{center}
\label{tbl:results}\\
\end{table}

\end{document}
